The widest definition of Oceania includes the entire region between continental Asia and the Americas, including islands in the Pacific Rim such as the Japanesearchipelago, Taiwan, and the Aleutian islands. On the other hand, the Oceania ecozone includes all of Micronesia, Fiji, and all of Polynesia except New Zealand.

For these reasons, it is not correct to say that Australasia is part of Oceania, because what is meant by 'Oceania' is not clear. Australasia has an exact definition in biogeography and geology. Australasia includes New Zealand, Australia (including Tasmania), and Melanesia, New Guinea, and the islands just north and east of Australia. All these are south-east of the Bali–Lombok line. This is known as the Wallace Line, after Alfred Russel Wallace.